The use of water, in various quantities and forms, as an additive to provide improved combustion of hydrocarbon fuels and waste gases is old.
Such processes have been proposed for various purposes: e.g. avoiding unsightly smoke from refinery waste-gas flares and improving the economics of home-type oil burner systems. These processes have included adding the water to the combustible as liquid, or as vapor, and in a very broad range of percentages.
Among the U.S. patents illustrative of such processes are those which utilize very high levels of water (U.S. Pat. No. 2,104,311) and very low levels of water (U.S. Pat. No. 3,862,819). Some patents disclose mixing of the water with the oil (U.S. Pat. No. 3,706,942) and some use the water in the form of a vapor catalyst -- often injecting the water as vapor. While some of the processes suggested in the art are probably of little value because they use too little or too much water, it may be safely assumed that, when operating at equilibrium, a substantial advantage in some combustion characteristics is achieved with many of the processes described in the art.
Nevertheless, substantial problems remain in implementing such processes commercially. The high combustion efficiencies to be realized are accompanied by the use of less secondary air. This means that more water of combustion is in a given volume of stack gas and undesirably high dew points are experienced. Simultaneously, the efficient high-temperature process in the furnace results in a highly efficient heat transfer in a properly designed furnace. This further decreases the amount of waste heat and also tends to increase the probability that a stack gas temperature will fall below its dew point.
Even when the dew point is generally maintained at a minimum practical level, any temporary fluctuation in draft conditions can cause serious condensation problems in the heat transfer and stack-gas handling portions of a heating system. What is required is a dependable, stable means to operate a water-catalyzed combustion process at something approaching a steady state. Problems relating to flame stability, to reignition, and even to the usual changes in draft air or in heating loads within a furnace, must be minimized. All this is ideally achieved without sacrificing any of the substantial combustion efficiency, which is attainable with H.sub.2 O-promoted combustion.